


instinctual

by ByTheBi



Series: AkuSaiMonth 2019 [1]
Category: Kingdom Hearts
Genre: Anal Sex, Berserker Isa, Explicit Sexual Content, Jealousy, M/M, Porn with Feelings, Romance, Teasing
Language: English
Status: Completed
Published: 2019-07-10
Updated: 2019-07-10
Packaged: 2020-06-25 19:04:13
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,443
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/19751911
Author URL: https://archiveofourown.org/users/ByTheBi/pseuds/ByTheBi
Summary: Reformed Isa still loves him, which is really all he could ask for, but he also treats Lea like glass, as if he might shatter if Isa’s too fierce in his affections. At first it’s sweet (Isa loves to remind him that Lea cried the first time they made love as humans with restored hearts), but sometimes Lea craves more.





	instinctual

**Author's Note:**

> A late contribution for Day 1 of Akusai month | Slow Burn  
> Today's theme is Change!

The strangest thing Lea learned after reuniting with his partner was that Isa wasn’t the same as Isa. 

Of course, he isn’t Saix either, not by a long shot, but he isn’t quite the boy Lea remembered. The newly restored Isa falls somewhere in-between: more jaded than the passionate boy of his youth and gentler than his cruel Nobody self. 

Reformed Isa still loves him, which is really all he could ask for, but he also treats Lea like glass, as if he might shatter if Isa’s too fierce in his affections. At first it’s sweet (Isa loves to remind him that Lea cried the first time they made love as humans with restored hearts), but sometimes Lea craves more.

Luckily for him, it turns out that Isa’s berserker state, though dampened, is still a thing for human Isa. And now that he’s a human with a full range of emotions, it turns out that the state is simply tied to strong feeling. 

The first time Isa loses control is during a full moon, when it’s hardest to resist the siren song of lunar power. He goes wild, pounding into Lea with an intensity that has the latter seeing the moon, sun, and stars as he climaxes.

When Isa apologizes profusely the next morning, returning to the same feather-light kisses that had become his usual fare, Lea just runs a hand over his bruises in delight, reveling in the fact that berserker Isa had at least him with evidence of their raptures. He hopes that Isa will leave him with new love marks before these fade, and he can’t help but feel disappointed when he doesn’t.

It isn’t for lack of trying on Lea’s part either. He brings up the subject a handful of times, purring into Isa’s ear how powerful and sexy he is when he’s moonstruck, how much faith Lea has in him to ravish him without hurting him. The words never seem to gain him more than a flicker of self-doubt in Isa’s eyes before he shuts the conversation down entirely.

Lea’s sly though, and more than a little stubborn. He learns quickly that Isa's short temper is the quickest way to get him to snap and he's always had a knack for pushing Isa's buttons; it's easy to make things go his way from there because he's pretty sure that deep down Isa wants to let loose too. So he has his fun, poking at various buttons throughout the lunar cycle until he figures out what combination unlocks the side of his boyfriend that's willing to throw him around, eager to leave marks on him that lasts for days afterwards. 

The morning after is always heartachingly tender, which isn't a bad thing on its own, but Lea is still working on wiping that guilty look on Isa's face when it happens. Most days he can kiss away Isa’s self-doubt, but there are some days where he sullenly withdraws and nothing Lea says will rouse him from it. While that should make Lea reconsider his frequent attempts at getting Isa to berserk, it only gives him new purpose. 

He is determined to get his lover to see that he has no reason to fear himself in that state. 

Lea puts the knowledge he has gained over the past few months to use. He plans his attack (or maybe it would be more fitting to call it sabotage) for the next full moon. It gives him plenty of time to prepare, much more than he needs actually, so there’s really nothing more for him to do other than sit and wait while the day draws near. 

There’s an energy in the air that day, though Isa goes about his morning unaware of it. He showers and dresses while Lea lazes in bed, watching his boyfriend through half-lidded eyes. Isa gives him a peck on the forehead goodbye but Lea drags him down by front of his shirt for a proper kiss. 

“Have fun at all your boring errands,” he says with a smug grin after he finally releases his boyfriend. 

Isa clears his throat, looking flushed. He smooths a hand down his shirt, frowning a little when he realizes that it wrinkled under Lea’s grip. “I was hoping to look presentable today, Lea.”

“Oh, were you?” He quickly passes a hand over Isa’s hair, ruffling it up before Isa can escape. 

“What are you-- Lea, stop it!” Isa snaps, finally pushing his hand away. He smooths his hair down with a huff, going over to the mirror to fix himself. “It’s too early for you to be this much of a troublemaker.”

Lea’s smile is cheeky. “Never too early for that.” Especially when his goal for the day involves ticking off Isa just enough to make it easier for him to lose it later. 

“You are incorrigible.”

“Isn’t that the way you like me?”

Isa doesn’t dignify him with a response, but Lea doesn’t really need one anyway. Instead he sidles up behind his boyfriend, wrapping his arms around his waist and kissing his ear. “Think of me while you’re out today,” he purrs because 

There’s a pause in which Lea can feel Isa struggle between giving into the sweetness or giving him a snarky reply. Eventually he turns in Lea’s hold, wrapping an arm around him and kissing him. 

“As if I could ever get you off my mind,” he mumbles against Lea’s lips. The words bring a genuine smile to Lea’s face. 

He can bug Isa more later. For now, he steals one last kiss before gently shooing him out the door before he’s late to his meeting with the Radiant Garden Restoration Committee to iron out some financial logistics on which they requested his input. 

For his part, Lea has no task or mission for the day, so he meets up with Ventus for lunch just to kill time. Ventus brings Terra along too, and the three of them spend hours chatting and catching up, wandering the town long after they finish their food. Every so often, Lea makes sure to text or even call Isa to pester him. The first few times he’s met with patience, but by the fourth unnecessary call, he’s met with an open exasperation that makes him grin. 

“Are you sure you don’t want to come back with us to say hi to Aqua?” Ventus asks at the end of the day, when they’re finally poised to go their separate ways. The sun is setting; the moon is little more than a ghost against the pink sky at this point, patiently awaiting its turn. The sight of it makes Lea eager to return home and await his moon’s arrival.

“Nah, maybe next time,” he says, waving his hand dismissively. “I wanna get home before Isa tonight.”

Terra and Ventus exchange a look, smiling. “Are you going to make him dinner or something?” 

“Something like that.” Lea shoots them a wide grin and bids them goodbye before finally setting off for home. 

By the time Lea gets home, there’s only half an hour left before he estimates when Isa will arrive. 

Lea parts the curtains. The moon hangs proudly in the sky, full of unearthly white light that cast a soft spotlight on the bed. He strips off his outer clothes and gladly takes his position in the center of the bed, body melting languidly into the pillows and sheets. He pulls out his phone.

After two rings, he’s met with the sound of Isa snapping into the receiver, “What is it now?”

“Hey, is that any way to greet your boyfriend?” Lea teases. His free hand roams across his stomach, following the trail of thin hair leading downwards. “Are you on your way home?”

“Yes, I am. Is that what you called me for?”

“Maybe I just wanted to hear your voice.”

“Maybe?”

Lea grins, pulling his boxers down and off to bare himself fully to the moonlight. He is already hard, skin hypersensitive in anticipation of what’s to come. “Definitely, baby.”

Isa’s scoff is annoyed, but not altogether unkind. “I’ll be home in a few minutes, Lea. We can talk then.”

He’s ignored. Lea grips his cock and, stroking lazily, demands, “Tell me about your day.”

There’s only a slight pause before Isa complies, his tone initially confused but eventually more lively as he rants about the incompetent people he’s met with today. Lea only half listens while he speaks, more focused on the sound of his voice than the actual words. 

“Are you listening?”

“Mmm, not really.”

The aggravated sound Isa makes has him smirking. He reaches over for the lube he set aside earlier and squirts some onto his fingers, humming and making vague noises of agreement while his boyfriend tells him off. He’ll have to make it up to him later.

“You literally asked me to tell you about my day and then you don’t listen? Tell me - why am I wasting my breath talking to you right now?”

“Because you love me?” The first finger goes in almost too easily. But Lea can’t complain, not when it feels so good.

He can practically hear the way Isa rolls his eyes. “I guess so.”

“Ah, come on, Isa,” he whines, and it’s only partially due to the exploratory finger he has inside of himself right then. “Don’t be mean.”

“I think your behavior today can qualify as ‘mean’ if we’re being realistic, Lea.”

Lea scoffs just as he adds as a second finger. “What do you mean, babe? I just wanted to hear your voice.”

“Yeah, sure.”

“C’mon, I’m sorry, Isa. Keep telling me about your day. Talk to me, honey,” Lea prompts. He’s still stretching himself, which only makes it all the more difficult to keep up with what Isa’s saying, but he makes an effort to follow along because while he wants to irritate his boyfriend, he’s incapable of neglecting him entirely. He listens while Isa fills him on all the boring details of his meetings, languidly stretching himself all the while.

“How was your day?” Isa asks eventually. There’s background noise on his end that tells Lea exactly what street he’s on. He pinpoints his boyfriend at about five minutes away. 

“Mmm, it was fine,” he answers. He pauses to catch his breath as one of his fingers grazes his prostate, stalling his brain. When his thoughts unfreeze, he continues, “I had lunch with some buddies today.”

“Oh really?” Isa hums pleasantly. “Who did you have lunch with?”

It takes him a minute to find his voice, too caught up in the act of working himself open. He’s not sure when he added a third finger, but he flexes all of them, feeling the delicious ache of his muscles stretching to accommodate him.

“Lea?” 

“I’m here,” he says after a moment’s pause. He blinks a few times, pressing the Gummiphone between his shoulder and his ear in an attempt to focus on the conversation at hand. 

“I asked you who you had lunch with.”

“Right, right.” But instead of answering, he just continues to open himself up, panting into the receiver and defecting questions, much to Isa’s frustration. 

“Are you going to tell me or are you just dragging this out just to be difficult?” Isa grumbled. There was a jingle of keys that signified he was getting close to home. Lea curled his fingers, sending spots dancing across his vision. He can’t contain his moan.

“Lea?” Isa sounds suspicious now. “What’s wrong?”

He doesn’t bother with an answer now that he knows his boyfriend is close. He focuses instead of the feeling of his slim fingers curling and flexing, easing himself open to the best of his ability. He’ll need all the help he can get for what he has planned tonight 

The front door to their apartment opens. “Lea, who did you see today?” Isa calls, right as he clicks end on their call. Lea drops the phone and waits. 

Their bedroom door swings open and Lea moans.

“Terra,” he breathes.

It’s no accident the lewd way he says the man’s name, and the way Isa’s shock sharpens into a narrow-eyed glare shows he knows it, too. 

“Lea, what-”

“It’s rude to interrupt someone when they’re talking, Isa.” Lea punctuates the statement with a pointed thrust of his fingers, gasping. He watches Isa undo his tie through half-lidded eyes. Excitement is coiling in his stomach.

“Haven’t seen him in a while but he’s looking good,” Lea purrs. 

The door closes behind Isa quietly. He’s abandoned his jacket and shoes already, leaving him in the same outfit Lea saw him in before he left his morning. Lea wants nothing more than to rip it off him.

“Is he now?” A single raised eyebrow, which is surprisingly calm coming from Isa. 

“Absolutely. He’s gained weight since you last saw him, but I wouldn’t be surprised if it was all muscle, honestly.”

Isa’s shirt is tossed onto the floor, a sharp contrast to the care he had shown it that same morning. He unbuckles his belt, scowling while he says, “Is that all you have to share about your day?”

“I could go on.”

“Don’t.” Amazing how a single word can hold so much authority. Emotions are brimming under the surface of Isa’s calm facade, and Lea can’t wait to pluck at them. 

For now, he sits up, crawling to the edge of the bed to meet his lover halfway and help him tug down his slacks. 

“Let me help you with that.”

Isa watches him with a furrowed brow, skeptical but doing nothing to stop him as he strips Isa of his pants. Lea begins pressing kisses to the bulge in Isa’s boxers, working it up to full mast with a teasing touch. 

It’s only when Isa’s erection is straining, twitching every time Lea so much as looks at it for too long, that he shifts gears. He slips a hand beneath the waistband of Isa’s boxers and gives him a few strokes. 

“Lea,” Isa groans, wallowing his head to fall down onto his chest. Lea takes that as a sign to continue, pulling Isa’s flushed cock entirely to give it a few ginger licks, prompting another groan from Isa. A hand tangles in Lea’s messy hair, possessive and insistent, but nothing beyond that. Lea follows his guidance, taking the head of his cock into his mouth. 

Isa’s arousal sits on his tongue in the form of salty precome. The taste isn’t entirely pleasant, but Lea laps it up anyway because it’s Isa’s and he knows the way he traces the slit of Isa’s cock with his tongue drives him wild. 

Except maybe not, because despite Isa’s panting, he isn’t doing anything. His hands are buried in Lea’s hair, but there’s no real strength to his grip, no tugging that would indicate he’s losing control. Lea sucks harder, running a hand up Isa’s cool thighs, but only gets a soft grunt in response. 

Well that won’t do at all. 

He pulls away entirely, making a show of laughing to himself as though the most hilarious thought in the world had just struck him. 

“What’s so funny?”

“Just remembered a funny joke Terra told me earlier.”

It’s a little mean to press on Isa’s jealous streak, but when a hand leaves his hair to grip his chin tightly, he can’t find it in himself to be sorry.

“Is that what you think about when blowing me?” Isa hisses. 

He shivers. “I mean, yeah, I guess it--”

Forceful fingers pry open his jaw, turning the rest of his sentence into gibberish. When he looks up, Lea notices that Isa’s glare is a harsh yellowish glow. 

He can’t smile with Isa grabbing his face the way he is, but the muscles in his cheeks try anyway, twitching beneath his hold. 

“Focus,” Isa snaps, and before Lea can come up with a sassy response to that, his mouth is being stuffed full of cock again.

Lea moans, saliva quickly pooling in his mouth and spilling out the sides when Isa pulls back and pushes back in with a harsh grunt. Isa fucks into his mouth without mercy, cock dragging over his tongue with such rapidity that he has no chance to tease, no opportunity to drag out his lover’s pleasure. Isa isn’t looking to last a long time anyway; Lea knows that when he gets like this, he seeks only to claim.

Which would be fine, especially since Lea would normally never pass up the opportunity to receive a load of Isa’s cum on his face, but tonight? Tonight he has plans for them.

He waits for his chance to speak, which comes when Isa’s frantic thrusting misses the mark and causes his dick to rub against Lea’s cheek instead of back into his hot mouth. Lea wastes no time, ignoring the smear of his own spit and Isa’s pre-cum on his face to groan, “Aren’t you gonna fuck me tonight?”

Isa closes his eyes and inhales deeply. The energy in the air is unreal, charged and powerful and intoxicating. 

When Isa opens his eyes again, it’s like lightning strikes. 

He lifts Lea like he weighs nothing and throws him back onto the mattress, making it creak in protest. Lea barely has to time to gather himself before Isa is on top of him, biting, kissing, bruising, leaving marks with extra sharp caninesL Lea runs his hands up Isa’s back, feeling the way the hard muscles shift beneath his fingers. His hands find their way to his shoulders and he anchors himself there, bracing himself for the pounding he’s sure to receive. 

And Isa does not disappoint. He pushes Lea’s knees up to his chest and slides into him in one smooth motion, bottoming out with a grunt. Lea’s earlier preparations make it as painless as possible, though nothing can ever prepare him for the sensation of Isa filling him or the harsh drag of his cock against his insides, hot and insistent. 

“God, yes,” he grinds out as Isa fucks him into the mattress. 

“Mine,” Isa snarls into his neck. His teeth are pressed against Lea’s throat and while there was once a time when that would have a dangerous place to be, now it only sends a thrill of lust shooting down Lea’s spine. A moonstruck Isa will bite him, mark him, leave hickeys scattered over his body, but he’s never hurt him. It’s thrilling to know that no matter how wild Isa gets, his feelings towards Lea are too deeply ingrained in him for him to ever forget.

“I’m all yours, baby,” he says, trying to sound reassuring, but all he can manage while Isa is pounding into him is a choked gasp. 

Isa seems to be pleased by that because he rolls his hips, grinding himself deeper into Lea just to hear him whimper before continuing his fast pace. Lea encourages him however he can, grinding his hips and letting out lewd moans. His fingernails dig tiny crescents into Isa’s shoulders, but it’s nothing compared to the purpling half-circles leaves on his skin.

A particular forceful slam of Isa’s hips drives him further up the bed, but he doesn’t time to react, not when Isa follows it up with a relentless series of thrusts that reduces them both to gapss and growls. 

“Isa,” he moans, lifting his hips to meet him. “Harder, babe, _please_.”

He can feel Isa’s chest rumble as he lets out a pleased rumble. There’s that familiar graze of teeth against his skin before Isa fucks into him with renewed vigor. For while there’s only the feeling of Isa - Isa filling him, Isa kissing him, Isa pressing his body against him. It’s dizzying how surrounded he is by Isa’s scent, but he has no complaints. This is exactly what he had been waiting for all day, and he’s drunk off the feeling of finally getting what he wants. 

“Mine, mine, mine,” Isa growls. His scar is jagged across the bridge of his nose, a harsh contrast to the open adoration shining in his eyes. He grinds into Lea, panting desperately. “All mine.”

“Yes, all yours, baby,” Lea agrees. At this point, he’s pretty sure he’d say anything to ensure Isa keeps fucking him like this -- hard and fast and eager, too excited for anything beyond the raw movement of their bodies, the slap of skin on skin, and the smell of sex in the air between them. 

Isa’s thrusts grow sloppier, somehow even more frantic at Lea’s affirmation. “Say it,” he demands, bending Lea’s body further against himself with every thrust.

“I’m yours!”

“More,” Isa demands, rocking into Lea’s hole. “Say you belong to me.”

“I do!” Lea cries out. Tears build in the corners of the eyes. “I belong to you, and you belong to me, Isa. ‘s just you n’ me, baby.”

It doesn’t take long after for Isa to reduce him into a tearful, drooling mess. He’s too far gone to feel shame, only away fo the Isa’s heart reconnecting with him, thumping so loudly that it can keep time for them both.

“Yes, Isa! Yes, yes! Fuck, I’m so close!”

Isa doesn’t bother replying to him; he never does in this state. Instead he pushes his face into the crook of Lea’s neck, pressing his nose to the underside of Lea’s face, inhaling his scent. His pace is still unrelenting, almost punishing in the way he drives into the Lea. Words are lost to him at this point anyway. Primal grunts and snarls make up his language, reminding Lea of how far gone he is. 

There’s no warning when Isa cums, only a sharp snap of his hips into Lea’s own and a powerful groan that rumbles through their joined bodies. Lea isn’t far behind, jerking himself off to completion while Isa milks the remainder of his climax by fucking into him a few more times before finally slumping over. 

Lea struggles to catch his breath, bringing his arms up to push his boyfriend off of him. “Hey, you’re heavy, y’know,” he says, grunting as he rolls Isa over onto his side.

Isa’s eyes are closed, his chest rising and falling softly with every breath he takes. Lea takes the opportunity to curl into his side after cleaning up the mess on his stomach before it grows tacky. He rests his head on Isa’s chest, smiling patiently while they both collect themselves.

This time when Isa opens his eyes, it’s green irises that look around. 

“Lea…” he begins, swallowing. He brings a hand up to his forehead, brushing his bangs back. “I don’t, I-I didn’t-”

“Hey, hey, hey,” Lea says immediately, rushing to cup his face, “it’s okay, Isa. You didn’t do anything I didn’t want you to do.”

“But I could have _hurt_ you.”

“You would never.” 

Isa’s eyes flicker down to Lea’s chest, where a prominent scar proves otherwise. His lips thin.

Lea grimaces, resisting the urge to cover the spot at which Isa stares. “Saïx would hurt me, but you, _Isa_ , would never.”

“How can you be so certain?” 

“How can you be so certain you’ll hurt me?” Lea shoots back, raising an eyebrow. 

Isa scowls. “Because it’s what I do. You should know better than anyone what lunar power does to me. I can’t control myself beyond my instincts, and that’s dangerous.”

“I guess it must be instinct for you to love me then.”

“W-What?” Isa’s face flushes. It’s not quite the confirmation Lea’s looking for, but it’s not a denial either, so he’ll take what he can get. 

“You say you only act on your instincts when you’re like that, but you haven’t hurt me since gaining your heart. So I feel like it stands to reason that your heart has some sort of pull on you even when you’re moonstruck. And since you love me, well…” Lea shrugs. “It makes sense that you’d never hurt me.”

Isa’s eyes flicker, searching Lea’s face for a trace of _something_ he can doubt, but Lea doesn’t give him the satisfaction. He meets his gaze head on, even going so far as to kiss his cheek when a few minutes pass. 

“How can you something like that without feeling embarrassed?” Isa says, burying his face into Lea’s hair to hide the redness in his own flushed cheeks.

“Because I love you,” Lea says as if it’s the simplest fact in the world. Perhaps, to him, it is. “And I know you love me too. And that’s why i trust you not to hurt me. You should give me the same courtesy. If I say I’ve got things under control, then you should show a more faith in me, y’know?”

“You make it sound so easy.”

“That’s because it is.”

He sighs. “Lea, if I hurt you--”

“You won’t.”

“But if I did, I don’t think I would be able to forgive myself, and you shouldn’t either.”

“Isa.” Lea tugs gently on his hair to get him to lock eyes with him. “You’ve _changed_ and that’s good. Sure, you might have some powers that teenage Isa might not have had, but you’re also a million times more considerate than Saïx ever was. You’re not your past, Isa. You’re your own future, and you ought to let me prove that to you.”

“I think you’ve done enough,” Isa says dryly, and the roll of his eyes is so light-hearted and fond that Lea can’t help but chuckle despite himself. 

He places a hand over Isa’s chest, feeling the rise and fall of his breath, the steady beat of his heart. All reminders of how alive they are. The moon, in all its brilliance, is shining down on them through the window, casting white light over them. Isa is never more beautiful than he is in these moments, messy and sleepy and shimmering under the moon’s ethereal glow. 

“I love you,” Lea says suddenly. His hand clenches into a fist over Isa’s chest. The words are more than a reminder; they’re a promise. 

“I love you, too,” Isa answers immediately. He doesn’t even need to think about what he’s saying. As though his body’s instinct really is to always love Lea. 

For his part, Lea only beams, so bright and delighted that Isa has no choice but to mirror him. Together, they outshine the feeble moonbeams spilling in from the window, truly radiant and strong in every sense of the word. 

(When they eventually nap, they both deny dreaming of a “next time” but that's okay because next time comes sooner and smoother than either of them could had hoped.)

**Author's Note:**

> I hope you enjoy this fic! I've been sitting on it for a while and it's nice to finally send it out in honor of akusaimonth. While I won't be participating every day this month, I do have a handful of things planned. I look forward to sharing them with you all <3
> 
> Find me on twitter @Bi_the_by for updates on my fics or even just to talk about ships!


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